1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods and apparatus for coupling a solder structure to a semiconductor chip input/output site.
2. Description of the Related Art
Flip-chip mounting schemes have been used for decades to mount semiconductor chips to circuit boards, such as semiconductor chip package substrates. In many conventional flip-chip variants, a plurality of solder joints are established between input/output (I/O) sites of a semiconductor chip and corresponding I/O sites of a circuit board. In one conventional process, a solder bump is metallurgically bonded to a given I/O site or pad of the semiconductor chip and a so-called pre-solder is metallurgically bonded to a corresponding I/O site of the circuit board. Thereafter the solder bump and the pre-solder are brought into proximity and subjected to a heating process that reflows one or both of the solder bump and the pre-solder to establish the requisite solder joint.
In one conventional process, the connection of the solder bump to a particular I/O site of a semiconductor chip entails forming an opening in a top-level dielectric film of a semiconductor chip proximate the I/O site and thereafter depositing metal to establish an under bump metallization (UBM) structure. The solder ball is then metallurgically bonded to the UBM structure by reflow. The opening in the dielectric film is shaped with relatively planar sidewalls, that is, without any protrusions or projections. One conventional example uses an octagonal opening. The later formed UBM structure has an interior wall that matches the planar sidewall configuration of the dielectric opening.
Flip-chip solder joints may be subjected to mechanical stresses from a variety of sources, such as coefficient of thermal expansion mismatches, ductility differences and circuit board warping. Such stresses can lead to crack propagation in the solder joint, particularly at the intermetallic interface between the UBM structure and the solder bump. In the conventional variant just described where the dielectric opening has a planar interior wall, cracks can propagate across the relatively open expanse between opposing or adjacent sidewalls of the UBM structure. Unimpeded crack propagation can lead to solder delamination and joint failure.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.